Conventional PO.sub.2 sensors comprise a pair of metal electrodes electrically connected by an electrolyte and encased in an oxygen permeable membrane in contact with the electrolyte and with the fluid to be examined. One electrode, the cathode, is of a metal capable of the electrochemical reduction of oxygen passing through the membrane so producing a current related to the oxygen partial pressure in the fluid.
Conventional PCO.sub.2 sensors also comprise a pair of electrodes, an electrolyte and a membrane, in this case permeable to carbon dioxide. The sensor functions by recording the effect of the change in pH resulting from the passage of carbon dioxide and water through the membrane, its dissolution to form carbonic acid and the dissociation of carbonic acid, liberating hydrogen ions according to the equation: EQU CO.sub.2 +H.sub.2 O.fwdarw.H.sub.2 CO.sub.3 EQU H.sub.2 CO.sub.3.revreaction.H.sup.+ +HCO.sub.3 -
A single unit carbon dioxide-oxygen sensing micro-electrode system is disclosed in Respiration Physiology 23 371-379 (1975). This system involves a platinum cathode and a silver-silver chloride anode in quinhydrone/potassium chloride electrolyte. This system can be used to measure PCO.sub.2 and PO.sub.2 independently, but not simultaneously.